BN Fallback v2 (#2080)

## Issue Addressed

- Resolves #1883

## Proposed Changes

This follows on from @blacktemplar's work in #2018.

- Allows the VC to connect to multiple BN for redundancy.
  - Update the simulator so some nodes always need to rely on their fallback.
- Adds some extra deprecation warnings for `--eth1-endpoint`
- Pass `SignatureBytes` as a reference instead of by value.

## Additional Info

NA

Co-authored-by: blacktemplar <blacktemplar@a1.net>

lcli/src/eth1_genesis.rs

@@ -15,7 +15,10 @@ pub const ETH1_GENESIS_UPDATE_INTERVAL: Duration = Duration::from_millis(7_000);
 pub fn run<T: EthSpec>(mut env: Environment<T>, matches: &ArgMatches<'_>) -> Result<(), String> {
     let endpoints = matches
         .value_of("eth1-endpoint")
-        .map(|e| vec![String::from(e)])
+        .map(|e| {
+            warn!("The --eth1-endpoint flag is deprecated. Please use --eth1-endpoints instead");
+            vec![String::from(e)]
+        })
         .or_else(|| {
             matches
                 .value_of("eth1-endpoints")